Add new "meta-keyword" `nullable`

Instead of this:

```yaml
foobar: "" # @schema type: string, null
```

it would be nice to use:

```yaml
foobar: "" # @schema nullable
```

which should be semantically identical.

This could combine with #247 to get a nicer experience, such as:

```yaml
foo: # @schema items/nullable
  - "bar"
  - true
```

